As Sorenson & Salamar argue Sarah sneaks off the ship into the jungle. The Doctor falls through a black void and meets with a creature. De Haan & Morelli remove Sorenson's samples but he conceals some in his cabin. Sarah calls for the Doctor at the black pit and is there when he is returned. Salamar is angered when Vishinsky halts take off to retrieve the Doctor & Sarah, who the occuloid tracker has spotted. In his quarters Sorenson begins a strange transformation as his eyes glow red, halted when he drinks of a solution. Vishinsky and De Haan revive the Doctor in the sickbay. The spacecraft again has trouble taking off but the Doctor gives up his sample, which enabled him to survive the trip to the other dimension, to Morelli to be jettisoned. However en route to the do this Morelli is killed. A mutating Sorenson drinks more of the solution to keep himself normal. Sorenson tries to convince them that another civilisation is trying to seize his discoveries. The Spaceship finds itself being pulled back to Zeta Minor, because the Doctor thinks there's still anti matter aboard. Salamar believes the Doctor and the Tardis are responsible. Sarah, left alone with Sorenson, watches transfixed as he begins to change and only comes to her senses when she hears De Haan's dying screams. She screams, and the Doctor overpowers Salamar and comes to investigate. Salamar stuns the Doctor, convinced he is responsible for De Haan's death and orders him ejected from the ship. Sorenson spills the last of a powder his solution is made from and begins to permanently transform. The Doctor & Sarah are locked in place and slide towards the opened ejection chamber.....

And the origins of this story stand revealed: It's Doctor Who's version of Jekyll & Hyde with the anti matter creature a direct lift from Forbidden Planet. Unfortunately I'm still having trouble engaging with the story. It's not doing anything for me. Sorry.

The background character Reig, who appears in this episode, is played by Melvyn Bedford will play a mummy in Pyramids of Mars, the next story screened but filmed immediately before this one.

The Doctor spends his career battling evil so it's no surprise that the word crops up in a few story titles:

The Aztecs 1: The Temple of Evil
The Daleks' Master Plan 3: Devil's Planet
The Evil of the Daleks
The Mind of Evil
Planet of Evil
The Face of Evil